In this grant, we propose to define the role of an important human-specific change in glycosylation, the loss of N-glycolylneuraminic acid (Neu5Gc), in Duchenne muscular dystrophy (DMD) and in Limb Girdle Muscular Dystrophy 2D. Neu5Gc is a form of sialic acid that is absent in all humans due to an inactivating mutation in the human CMAH gene. By contrast, Neu5Gc is an abundant form of sialic acid in heart and skeletal muscles in almost all other mammals besides humans, including the great apes. To assess the role of human CMAH in DMD, we have created Cmah-/-mdx mice, in effect humanizing this aspect of mouse glycosylation. The mdx mouse has been studied for several decades as a model for DMD, a relatively common and ultimately fatal X- linked neuromuscular disorder. Despite mimicking loss of dystrophin, the gene defective in DMD, in almost all muscle cells, mdx mice do not show the mouse equivalent of a pediatric presentation of disease. Children with DMD show loss of ambulation, usually by 12 years of age, followed by respiratory and/or cardiac failure and death usually in the third decade of life. mdx mice, by contrast, show little change in these same features until near the end of their normal lifespan, which is typically reduced by only 1-2 months relative to wild type animals. Cmah-/-mdx mice, by contrast to mdx animals, show an 88% deficit in diaphragm muscle strength and a 66% deficit in cardiac trabecular muscle strength, compared to wild type, by 8 months of age, with half of animals dying by 11 months. Such mice also show significantly impaired ambulation relative to mdx. The early and robust presentation of these phenotypes, which are the primary drivers of DMD morbidity and mortality, in a small animal model that carries a genetically appropriate human-like change in the mouse genome will be a great asset to translational research to identify therapies for DMD and other human diseases. In Aim 1 of this proposal, we will investigate both the loss of function aspects to Neu5Gc deficiency as well as gain of function immune aspects as they relate to disease severity in Cmah-/-mdx mice. In Aim 2, we will devise therapies to offset the increased disease severity resulting from deletion of Cmah that could be translated into therapies for muscular dystrophy patients. In Aim 3, we will study the role of Cmah in a second mouse model of muscular dystrophy, the Sgca-/- model of Limb Girdle Muscular Dystrophy 2D. These experiments will investigate the cause of increased disease severity resulting from Cmah deficiency, assess the generality of Cmah deficiency in altering disease, and develop therapies to offset the loss of human CMAH that could ameliorate muscular dystrophy in DMD and LGMD2D patients.